1. Field of the Invention
The invention relates to an apparatus for liquefying a thermoplastic plastic, in particular an adhesive on the basis of polyurethane.
2. Description of the Prior Art
In such a liquefying apparatus as is known from DE-GM 86 00 021 (U.S. Pat. No. 4,724,983) the heatable sealed supply container comprises an upper portion for receiving a downwardly open transport container for the thermoplastic plastic and a lower portion serving as buffer volume. A heating means heats the thermoplastic plastic disposed in the transport container so that said plastic becomes liquid and drops down into a base portion which is disposed beneath the supply container and the transport container and receives a collecting trough for the liquefied thermoplastic plastic. At the upper end of the supply container a pivotally constructed cover is disposed which is provided at its inside with a mandrel; when the cover is closed the tip of the mandrel penetrates into the upwardly directed face of the transport container.
An inert gas usually serving as protective gas is supplied to the interior of the supply container via a conduit.
However, in tests with such a liquefying apparatus problems have been encountered related mainly to the supply of the gas, for example nitrogen, carbon dioxide or also compressed air in the case of less sensitive plastics. Firstly, there is the danger that the gas collects only in the base portion of the liquefying apparatus and in particular does not reach the thermoplastic plastic in the transport container so that in the latter undesirable reactions can occur. In spite of this screening action being not satisfactory, the gas consumption is very high because gas has to be continuously supplied to the interior of the supply container.